In recent years, LAN (Local Area Network) is becoming more important. In such network, a plurality of communication stations connected to the network share one medium in order to transmit packets. However, in the case in which a plurality of transmission stations transmit the packets at the same time, the packets collide with each other. Here, it becomes necessary to define a mechanism for effectively avoiding the collision.
For example, a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) based collision avoidance method; called DCF (Distributed Coordination Function), is defined in the IEEE 802.11 wireless communication method (conforming to the ANSI/IEEE Std 802.11, 1999 Edition) which is the reference for Wireless LAN.
In such conventional network, transmission rights are equally granted with respect to all the transmission stations. Therefore, in the case in which the total amount of traffics flowing in the network is increased, the bandwidth for each stream is reduced. This will be a problem for flowing real time stream data, such as videos and voices each of whose transmission delay time is limited. That is, such stream data is not transmitted properly in the case in which the network is congested.
In order to properly transmit the stream data, various mechanisms for securing the bandwidth have been developed. As shown in FIG. 5, one method for securing the bandwidth is that a central control station 102 in the network manages a part of the bandwidth necessary for transmitting data from a transmission station (communication station) 100 to a reception station (communication station) 101. In such method, (i) a transmission station notifies the central control station of information concerning a traffic property of the stream data to be flown through the network, (ii) the central control station judges whether or not the transmission of the stream data is acceptable, and then (iii) if it is judged that the transmission of the stream data is acceptable, the central control station grants the transmission right to the transmission station.
Regarding the IEEE 802.11 wireless communication method, a subgroup called TGE is discussing a function of the central control station, the function called HCF (Hybrid Coordination Function) for managing the bandwidth in the wireless network. According to the draft (conforming to IEEE Std 802.11e/D3.3, 2002) designed by TGE in the conference held on September, 2002, the central control station called an HC (Hybrid Coordinator) manages a part of transmission rights of the traffics of the transmission stations belonging to the network. The communication stations other than the HC are called WSTA (Wireless Station).
A WSTA notifies the HC of information concerning the traffic property of data to be transmitted from the WSTA itself or information concerning a polling request specification. The information is called Traffic Specification (TSPEC). The information concerning the traffic property of data is, for example, minimum/average/maximum data rates of the traffic, a tolerable transmission delay time, and the like information, and the information concerning the polling request specification is, for example, minimum/maximum time intervals between two successive times at which polling is desired, and the like information. Major parameters of the TSPEC currently defined in Draft 3.3 are as follows:
TSInfo ACK Policy indicates (i) whether or not ACK (acknowledgement) is necessary and (ii) a desired form of ACK, as follows: “00” indicates “Normal ACK”, “10” indicates “No ACK”, “01” indicates “Alternate ACK”, and “11” indicates “Group ACK”;
Direction specifies as follows: “00” indicates “Up link”, “10” indicates “Down link”, “01” indicates “Direct link”, and “11” indicates “reserved”.
Minimum Data Rate specifies a lowest data rate (in units of bps) of a traffic. The Minimum Data Rate does not include MAC/PHY Overheads. “0” indicates that the Minimum Data Rate is not specified.
Mean Data Rate indicates an average data rate (in units of bps) of the traffic. The Mean Data Rate does not include the MAC/PHY Overheads. “0” indicates that the Mean Data Rate is not specified.
Peak Data Rate indicates a maximum allowable data rate (in units of bps) of the traffic. “0” indicates that the Peak Data Rate is not specified.
Max Burst Size indicates a maximum data burst (in units of octets) of the traffic that arrives at Peak Data Rate. This is a parameter for a variable rate traffic or for a burst traffic. A value of 0 indicates that there are no bursts.
Nominal MSDU Size indicates a normal size (in units of octets) of MSDU. The MSDU size indicates the size of data transmitted/received from/to an upper layer to/from the MAC layer. Moreover, the MSDU size is equal to a length obtained by subtracting headers of the MAC and physical layers from the packet. An MSDU Size of 0 indicates that the Nominal MSDU size is not specified.
Inactivity Interval indicates a maximum amount of time (in units of μs) that may elapse until a connection is cut by the central control station in the case in which the traffic of MSDU is not flowing. If the inactivity interval is “0”, the connection will be continued without Inactivity interval.
Delay Bound indicates a maximum amount of time (in units of μs) allowed to transport an MSDU belonging to the traffic. “0” indicates that the Delay Bound is not specified.
Min PHY Rate indicates a minimum physical rate (in units of bps) of the traffic. “0” indicates the Min PHY Rate is not specified.
Minimum Service Interval indicates a minimum value of an interval between time points when the transmission right for a traffic is granted. In the case in which the Direction field is set to Uplink/Sidelink, the HC carries out polling. Therefore, this parameter indicates a minimum interval (in units of μs) between the start time of two successive QoS CF-POLLs (will be described later). a communication station that wants to save power sets this parameter.
Maximum Service Interval indicates a maximum value of an interval between time points when the transmission right for a traffic is granted. In the case in which the Direction field is set to Uplink/Sidelink, the HC carries out polling. Therefore, this parameter indicates a maximum interval (in units of us) between the start time of two successive QoS CF-POLLs (will be described later).
Surplus Bandwidth Allowance Factor indicates the excess allocation of time (bandwidth) over and above the rates required to transport an MSDU belonging to the traffic. This field indicates a ratio of over-the-air bandwidth, including retransmissions and the MAC/PHY overheads, to bandwidth of the transported MDSUs required for successful transmission.
In order to meet requests from the WSTAs, the HC having received TSPEC from each WSTA carries out a calculation (scheduling) for determining an order and time periods for granting the transmission rights with respect to those stations that are to carries out transmission. Then, based on results of the scheduling, the HC grants the transmission right with respect to the WSTAs.
A transmission right granted time period granted by the HC to a station is called a TXOP (Transmission Opportunity). The HC transmits a packet called QoS CF-POLL to the WSTA to which the transmission right is about to be granted. In this way, the TXOP is granted to each transmission station. The QoS CF-POLL packet contains information concerning a time limit within which the transmission right is granted, the information called a TXOP LIMIT. The WSTA to which QoS CF-POLL is transmitted is allowed to transmit data within the time limit.
A unit of data that an upper layer of a communication station requests the MAC layer of the communication layer to transmit is called an MSDU (MAC Service Data Unit). Actual transmission of the MSDU through a medium is carried out by transmitting the MSDU in a form of a packet. The packet is normally formed by adding protocol headers of the MAC layer and the physical layer to one MSDU.
The current draft defines a method using Normal ACK and a method using Group ACK as a method of giving the acknowledgement from the reception station to the transmission station in transmitting data from the transmission station to the reception station. FIG. 6(a) is a diagram showing the method using Normal ACK, and FIG. 6(b) is a diagram showing the method using Group ACK. As illustrated in FIGS. 6, according to the method using Normal ACK, each time the transmission station transmits a packet 110, acknowledgement (ACK) 111 with respect to the packet 110 returns from the reception station. Meanwhile, according to the method using Group ACK, the transmission station transmits a plurality of packets 111 to the reception station in a burst manner. Then, in the case in which the transmission station transmits a packet 112 called Group ACK Request and the packet 112 is received by the reception station, the reception station returns a packet 113 to the transmission station, the packet 113 called Group ACK including the acknowledgements with respect to the packets which have been transmitted from the transmission station.
The number of packets transmitted according to a burst transmission is not necessarily a fixed number. As a typical sequence, considered here is a pattern in which a fixed number of packets (N packets in FIG. 6) are periodically transmitted by the burst transmission as shown in FIG. 6. Here, the number N is called a burst length.
By using the method of Group ACK, the acknowledgements with respect to a plurality of packets can be notified to the transmission station at one time. On this account, using the method of Group ACK has a better efficiency for bandwidth utilization than using the method of Normal ACK. In addition, the longer the burst length N is, the better the efficiency for bandwidth utilization becomes. However, the longer the burst length N becomes, the lower the frequency of returning the acknowledgements becomes, and thus the number of times the same packet can be retransmitted within a certain time period is reduced.
For example, in the case of using a physical layer conforming to IEEE 802.11a, a method of calculating a time period necessary for transmitting one packet by using Normal ACK is as follows: in the case in which a packet is a QoS Data packet, there are two parameters: MSDU size: L (bit) Physical rate: R PHY. A N DBPS (a number of bits transmitted per symbol) in OFDM used in the physical layer conforming to IEEE 802.11a is determined from a RPHY (physical rate), as shown in FIG. 7. Therefore, N SYM (the number of OFDM symbols necessary for transmitting L (bit) MSDU) can be obtained by the following formula:N SYM=ceiling {(310+L)/N DBPS}.A TQoSData (time period necessary for transmitting one packet) can be obtained by the following formula:TQoSData=20+4×N SYM. (μs)
As for an ACK packet to be returned by the reception station after receiving the QoSData packet, the physical rate R PHY (ACK) of the ACK packet is determined according to the physical rate of the QoSData packet. Moreover, the value of N DBPS is determined according to R PHY (ACK) (see FIG. 7). The number N SYM of OFDM symbols necessary for transmitting the ACK packet can be calculated by the following formula:N SYM=ceiling (134/N DBPS).Therefore, a T ACK (time period for transmitting one packet) can be obtained by the following formula:T ACK=20+4×N SYM. (μs)
According to the above calculations, a reference time period T normal (L, R PHY) necessary for exchanging the QoSData packet and the ACK packet can be obtained by the following formula:T normal (L, R PHY)=TQoSData+SIFS+T ACK+SIFS, (μs)where SIFS is a gap time between two successive packets. The SIFS is 16 (μs) concretely in the case of using the physical layer conforming to IEEE 802.11a.
Similarly, a method of calculating an average time period necessary for transmitting one packet by using Group ACK is as follows.
The time period TQoSData necessary for transmitting the QoS Data packet can be obtained by the same formula as that of the case of Normal ACK.
As for a time period necessary for transmitting a Group ACK Request packet, because the physical rate R PHY (GAR) of the Group ACK Request packet is determined according to the physical rate R PHY of the QoSData packet, the N DBPS corresponding to R PHY (GAR) is obtained according to FIG. 7. Moreover, the T GAR (time period for transmitting one packet) is determined according to the following formulas:N SYM=ceiling (214/N DBPS),T GAR=20+4×N SYM. (μs)
Meanwhile, as for a time period necessary for transmitting the Group ACK packet, because the physical rate R PHY (GA) of the Group ACK packet is determined according to the physical rate R PHY of the QoS Data packet, the N BPS corresponding to R PHY is obtained according to FIG. 7. Moreover, the T GA (time period for transmitting one packet) is determined according to the following formulas:N SYM=ceiling (1238/NDBPS),T GA=20+4×NSYM. (μs)
A reference time period T group (N) (L, R PHY) necessary for exchanging N QoS Data packets and the Group ACK Request/Group ACK packet can be obtained by the following formula:T group (N)(L, R PHY)=N·TQoSData+SIFS+T GAR+SIFS+T GA+SIFS. (μs)A reference value T group (L, R PHY) of an average time period necessary for transmitting one packet in the case of transmitting the packet by using a Group ACK sequence having the burst length N can be obtained by the following formula:T group (L, R PHY)=T group (N)/N. (μs)
From the above calculations the parameters for transmitting data by using Group ACK can be obtained from the MSDU size of the packet, the physical rate for transmitting the packet and the burst length N.
In the case of the transmission using Group ACK, the efficiency for bandwidth utilization changes according to the burst length N. Therefore, an hour rate of the TXOP allocated by the central control station to the stations changes. However, because the communication station is allowed to send a Group ACK Request/Group ACK packet at any time, the concept “burst length” does not exist in the specification, and a mechanism of transmitting the burst length to the central control station is not defined, either. Instead, a control station can notify the central control station of Surplus Bandwidth Allowance of TSPEC. Surplus Bandwidth Allowance is information corresponding to a rate of a bandwidth (or an average time period for transmission), which is expected to be actually necessary, to the bandwidth (or an average time period for transmission) of the case of using Normal ACK. The following is a method of calculating Surplus Bandwidth Allowance transmitted to the central control station from the communication station which uses Group ACK and transmits the packets of the burst length N. That is, the value of Surplus Bandwidth Allowance (A surp) required to be set in TSPEC is obtained by the following formula:A surp=T group (L, R PHY)/T normal (L, R PHY).
Although the current draft does not state so clearly, there is a possibility that a rule will be set in which the transmission station calculates an additional bandwidth (or additional transmitting time) necessary for retransmitting the packets, and creates Surplus Bandwidth Allowance (A surp) with the additional bandwidth included therein so as to apply for a value taking the additional bandwidth in consideration. In the case in which PER indicates the packet error rate, generally, a bandwidth multiplied by 1+PER+PER2+PER3+ . . . =1/(1−PER) becomes necessary. Therefore, the value of Surplus Bandwidth Allowance which takes into account the bandwidth for retransmission can be obtained by the following formula:A surp′=T group (L, R PHY)/T normal (L, R PHY)/(1−PER),where the value of PER in the above formula may be the packet error rate actually measured by the communication station in the past communication, or may be a fixed value (typical value).
By way of example, FIG. 8 illustrate sequences for exchanging the packets in the case of actually carrying out the communication by using Normal ACK or Group ACK within the TXOP granted by the central control station. FIG. 8(a) is a diagram illustrating the method of using Normal ACK, and FIG. 8(b) is a diagram illustrating the method of using Group ACK. Here, a cycle of an average time period in which a sequence of n packets, Group ACK Request, and Group ACK is transferred through a medium is referred to as an average burst output cycle (T burst).
As a conventional technology, there is “Draft Supplement to REFERENCE FOR Telecommunications and Information Exchange Between Systems—LAN/MAN Specific Requirements—Part 11: Wireless Medium Access Control (MAC) and Physical Layer (PHY) specifications: Medium Access Control (MAC) Enhancements for Quality of Service (QoS), IEEE Std 802.11e/D3.3, 2002.”
The network system described above has such a mechanism that the transmission stations only transmit the properties of the streams and the central control station “handles all” the allocation of the transmission rights. Therefore, it is impossible to guarantee that a transmission period having a timing and a length as expected by the transmission station is assigned to each transmission station.
For example, even in the case in which, in order to transmit a traffic with a fixed rate, the transmission station requests the central control station for a transmission period with such property parameters regarding minimum/average/maximum data rates of the stream that the property parameters regarding minimum/average/maximum data rates are of the same value, there is a possibility that the central control station does not grant the transmission right to the transmission station periodically. Because the central control station accepts various requests from many transmission stations at the same time, the central control station usually grants the transmission right nonperiodically when focusing on the transmission right allocation with respect to each stream.
For example, reference numerals 130 to 133 in FIG. 9 show results of various scheduling carried out by the HC. A reference numeral 120 indicates a QoS CF-POLL packet, and a reference numeral 121 indicates the TXOP granted with respect to the transmission station. Thus, even if the HC accepts identical TSPECs from the transmission stations, the HC may grant various TXOPs. The current draft does not define how much observation time is necessary for the central control station to provide the transmission time period corresponding to the requested data rate. Therefore, this depends on implementation. In other words, the method defined in the current draft gives great flexibility with respect to the scheduling of the HC.
The following explains that reliability of the communication path varies according to how to grant the transmission right by the central control station. In FIG. 10, horizontal stripes are drawn in those TXOPs assigned to the polling operations within a certain time period, the polling operations shown in FIG. 9. It is clear from FIG. 10 that the amount of TXOPs, granted in a time period from T1 to T2, of the polling 133 is less than that of each of the polling operations 130, 131, and 132. When considering a case of making the stream flow with a fixed rate, the amount of the transmission right granted in the time period from T1 to T2 in the polling 133 is less than an average amount of the transmission right. Therefore, at a time point T2, a large number of not-yet-transmitted MSDUs (MAC Service Data Unit) remain in a transmission buffer. If the upper layers request to transmit another MSDU 140 at the time point T2, the MSDU 140 is placed in a last of a queue in the transmitting buffer. Therefore, in the case of a polling like the polling 133, it takes a long time until a first packet of the MSDU 140 is transmitted. It is necessary for all the packets to be transmitted to the reception station within the same tolerable transmission delay time. However, in the case in which the time period from T1 to T2 and the time period from T2 to T3 is equal to the tolerable transmission delay time, it is clear that the opportunity of the retransmission of the MSDU 140 in the polling 133 is less than that of each of the polling operations 130 to 132.
Less opportunity of the retransmission indicates that the packet loss rate (PLR) becomes high. Here, the packet loss rate denotes a ratio of packets which are not delivered to the reception station in a time limit (that is, in the tolerable transmission delay time) at an end of repeated retransmission of the packets from the transmission station to the reception station.
Especially in the case of carrying out the communication by using a mechanism in which the transmission station transmits a plurality of packets to the reception station in a burst manner and the reception station notifies, at one time, the transmission station of the acknowledgements with respect to a plurality of the received packets, the frequency of the retransmission is low. Therefore, the packet loss rate is seriously affected by the difference among the polling operations. By frequently notifying the transmission station of the acknowledgements, the difference between the packet loss rates of the polling operations can be reduced. However, in the case in which the acknowledgements are notified too frequently, the efficiency for bandwidth utilization is deteriorated. Because the mechanism of the burst transmission is designed to increase the efficiency for bandwidth utilization, it is preferable that the desired packet loss rate be achieved without deteriorating the efficiency for bandwidth utilization. However, the current draft does not include a guideline concerning how often the transmission station should request the acknowledgements with respect to the reception station.
FIGS. 15 to 18 show concrete examples of various methods of granting the transmission rights by the central control station. Note that, the examples assume that the central control station grants the transmission rights in a 100 TU cycle, 30 MSDUs are inputted in one cycle and are spaced equally, and the maximum tolerable transmission time of each MSDU is 50 TU (here it is assumed that 1 TU=1024 us). The numerals in the TXOPs in the figures indicate the number of packets outputted in the respective TXOPs. In the allocations of TXOPs in Examples 1 to 4, the transmission rights are so granted that 36 packets are outputted in 100 TU.
In order to consider various patterns of the allocation of TXOPs, a variable is used in each allocation of TXOPs. In the allocation of TXOPs shown in FIG. 15 (Example 1), the transmission rights are uniformly granted so that x MDSUs are successively transmitted, and x is a variable here.
In the allocation of TXOPs shown in FIG. 16 (Example 2), the transmission rights are not granted in t TU of 100 TU, and the packets are uniformly transmitted in the rest of 100 TU. Moreover, the transmission rights are so granted that three MSDUs are successively transmitted, and t is a variable here.
In the allocation of TXOPs shown in FIG. 17 (Example 3), the transmission rights are uniformly granted so that three MSDUs are successively transmitted in t TU and six MSDUs are successively transmitted in (100−t) TU. t is a variable here.
In the allocation of TXOPs shown in FIG. 18 (Example 4), the transmission rights are uniformly granted so that eighteen MSDUs are successively transmitted in t TU and three MSDUs are successively transmitted in (100−t) TU. t is a variable here.
FIGS. 19 to 22 show results of simulations of how the packet loss rate changes in the concrete methods of granting the transmission rights by the central control station as illustrated in FIGS. 15 to 18. The horizontal axis of each figure indicates the variable in the allocation of TXOPs. The value of the maximum transmission delay time is also shown in the figures. (Because the maximum tolerable transmission time is 50 TU here, the packet with maximum transmission delay time exceeding 50 TU causes packet loss.) According to these figures, even in the case in which the sum of TXOPs allocated in a certain time period is the same, the maximum transmission delay time and the packet loss rate vary according to the method of granting the transmitting rights.
Each stream application has a tolerable packet loss rate with respect to the communication path. Some applications properly operates when PLR=10−4, and others may require PLR=10−8. However, the current draft does not include a method in which the transmission station gives, to the central control station, information concerning the packet loss rate desired by the application of the transmission station with respect to the communication path. Therefore, in the communication path, such as a wireless communication path in which errors occur so often, the transmission station needs a method for transmitting to the central control station the information necessary for achieving the desired packet loss rate.
Even in the case in which the video is distorted in the reception station because the transmission rights is allotted by the central control station in improper timing, a user thinks that the transmission station or the reception station is broken. This is not unfavorable for manufacturers of transmission stations and reception stations. In the current draft, the transmission station can transmit not only the information concerning “stream property” but also the information concerning “the polling request”. However, this is not enough. Moreover, the current draft does not include a guideline concerning how to set each parameter.
The present invention was made to solve the above problems, and an object of the present invention is to provide a communication management method, a central control station, a communication station, a communication management program, and a computer-readable recording medium storing the communication management program each of which can especially realize, in a communication network in which the packet error rate is comparatively high, the communication path quality requested by the transmission station to the communication path while maintaining flexibility of the scheduling carried out by the central control station.